The phenomenon of laser-induced ablation transfer imaging is generically known and is believed to entail both complex non-equilibrium physical and chemical mechanisms. Such laser-induced ablation transfer is thought to be effected by the rapid and transient accumulation of pressure beneath and/or within a mass transfer layer initiated by imagewise irradiation. Transient pressure accumulation can be attributed to one or more of the following factors: rapid gas formation via chemical decomposition and/or rapid heating of trapped gases, evaporation, photo expansion, thermal expansion, ionization and/or by propagation of a pressure-wave. The force produced by the release of such pressure is sufficient to cause transfer of the imaging layer to an adjacent receptor element. The force is preferably sufficient to effect the complete transfer of the exposed area of a layer rather than the partial or selective transfer of components thereof.
Laser-induced thermal mass transfer of materials from a donor sheet to a receptor layer has been described in the patent and technical literature for nearly thirty years. However, few commercial systems have utilized this technology. Exposure fluences required to transfer materials to a receptor have been, at best, on the order of 0.1 Joule/cm.sup.2 (i.e., J/cm.sup.2). Consequently, lasers capable of emitting more than 5 Watts of power, typically water-cooled Nd:YAG lasers, have been required to produce large format images (A3 or larger) in reasonable times. These lasers are expensive and impractical for many applications. More recently, single-mode laser diodes and diode-pumped lasers producing 0.1-4 Watts in the near infrared region of the electromagnetic spectrum have become commercially available. Diode-pumped Nd:YAG lasers are good examples of this type of source. They are compact, efficient, and relatively inexpensive. In order to use these new sources in a single-beam, large format imaging system, the exposure fluence of thermal transfer materials should be reduced to less than 0.04 J/cm.sup.2 and the exposure pixel dwell time should be less than 300 nanoseconds. There have been many unsuccessful efforts in the art to achieve this goal.
Recently, however, U.S. Patent application Ser. No. 07/977,215, filed Nov. 16, 1992 and entitled "PROPELLANT-CONTAINING THERMAL TRANSFER DONOR ELEMENTS," disclosed a thermal transfer donor element containing a gas-producing polymer having a thermally available nitrogen content of greater than about 10 weight percent, a radiation absorber, and a thermal mass transfer material. Such gas-producing polymers generate a high propulsive force, thereby decreasing the exposure fluence required to induce transfer of imaging material to a receptor layer material. For this reason, the gas-producing polymers enable the use of simple, single-beam scanners based on diode-pumped lasers such as diode-pumped Nd:YAG lasers.
Generally, three types of radiation absorbers are used in thermal mass transfer imaging systems: dyes, particles, and thin layers of metal. The use of dyes as a radiation absorber is disclosed in U.S. Pat. No. 5,156,938. In this role, however, dyes are undesirable because of their high cost, reactivity/incompatibility with other components of the thermal transfer system (which, in turn, leads to instability and a low shelf life), and susceptibility to decomposition under the high temperature conditions which exist during thermal imaging.
Particle-type radiation absorbers are disclosed in, e.g., U.S. Pat. No. 4,588,674, UK Patent Application GB 2 083 726, and Japanese Kokai Patent Application No. SHO 63-161445. Such particles are generally dispersed in a binder. The most common particle-type radiation absorber is carbon black. Because the particles are discrete and randomly distributed in the binder, they must be present in relatively thick (i.e., greater than 0.5 micrometers) layers in order to generate sufficient heat for mass transfer. Since the amount of radiant energy required to heat a layer is directly proportional to the thickness of that layer, however, such thick layers are undesirable from both a speed and energy usage standpoint. In addition, when carbon black is used as a particle absorber (which is typically the case), the persistent color of the particles generally restricts their use to thermal mass transfer systems which are black and white.
Thin-layered metal absorbers avoid the disadvantages of dye and particle absorbers by combining low cost, high compatibility and high stability with the ability to provide sufficient heat for mass transfer when coated in thin (i.e., around 0.1 to 0.01 micrometers) layers. In this manner, thin-layered metal radiation absorbers increase the efficiency of the imaging process by allowing greater speed and lower energy usage. For example, copending U.S. patent application Ser. No. 08/033,112, filed Mar. 18, 1993 and entitled "LASER PROPULSION TRANSFER USING BLACK METAL COATED SUBSTRATES," discloses a thermal transfer donor element containing, in order, a substrate, a black metal radiation absorbing layer on one surface of the substrate, a gas generating polymer layer over the black metal layer, and a colorant over the black metal layer. The donor element is particularly useful for ablative thermal mass transfer imaging.